1. Field of the Invention
The present invention relates to a preparing method for a nickel hydroxide used as an active material for a nickel electrode, operating as a positive electrode in an alkaline secondary battery, a manufacturing method for the nickel electrode using this nickel hydroxide, and an alkaline secondary battery incorporating this nickel electrode as its positive electrode therein. More specifically, the invention relates to a preparing method for a nickel hydroxide adapted for use as an active material for a positive electrode, operating as a positive electrode in an alkaline secondary battery and enjoying satisfactory discharge characteristics and cycle life performance.
2. Prior Art
Recently, nickel-hydride secondary batteries have become the object of public attention as high-capacity alkaline secondary batteries. The batteries of this type act with hydrogen as an active material for a negative electrode, and have negative and positive electrodes arranged in layers with a separator between them. The negative electrode is formed of a current collector and a hydrogen-occlusive alloy which can reversibly occlude and release hydrogen, and which is supported on the current collector, positive electrode is formed of a similar current collector and nickel hydroxide as an active material for a positive electrode, which is supported on the current collector. The negative and positive electrodes constitute an electricity generating element, which is disposed in an alkaline electrolyte, such as potassium hydroxide, and the whole structure is sealed.
A sintered nickel electrode has been the prevailing material for the positive electrode of these batteries. In this nickel electrode, a current collector made of a sintered body of nickel powder is filled with an active material compound which consists mainly of nickel hydroxide which is an active material for a positive electrode. In this case, however, the packing density of the activating material compound is low because the sintered nickel material used as the current collector has a low porosity. Accordingly, this nickel electrode cannot effectively meet the recent demand for higher-capacity alkaline secondary batteries.
Recently, therefore, the following nickel electrode has been developed for use as a positive electrode.
The nickel electrode is formed as follows. A powder of nickel hydroxide, conductive material powder such as nickel powder or cobalt powder, and a binding powder such as polytetrafluoroethylene powder are mixed in predetermined ratios. A given amount of a thickener such as an aqueous solution of carboxymethylcellulose is added to the resulting powder mixture to prepare an active material paste. A current collector with high porosity, e.g., a spongy expanded nickel plate, is filled with the active material paste. Thereafter, the whole resulting structure is dried and pressurized so that the individual powders are bound together by means of the binder and carried in the current collector. Since the current collector used for this nickel electrode has a high porosity, the packing density of the active material compound is high.
If the amount of fill of the active material paste in the current collector is increased a higher-capacity battery results. However, in the case of such a nickel electrode, repeated operation of charge and discharge of the battery which incorporates the nickel electrode is liable to cause deformation of the nickel electrode as a whole, and is liable to cause swelling of the supported active material paste.
In such a situation, the electrical connection between the nickel hydroxide as a active material for a positive electrode and the current collector is worsened, so that the discharge capacity and discharge voltage which can be obtained from the battery are reduced. As the positive electrode (nickel electrode) is deformed or expanded, moreover, the distribution of the electrolyte, having so far been properly balanced between the positive electrode, separator, and negative electrode, is ruined inevitably. The electrolyte contained in the separator, in particular, moves to the positive electrode side, so that its quantity in the separator is reduced. Thus, problems arise such that the internal resistance of the battery increases, and the cycle life characteristic of the battery is shortened.
In order to solve these problems, there has been proposed, in Jpn. Pat. Appln. KOKAI Publication No. 2-30061, for example, a method in which ions of other metals than Ni, such as Zn and Cd, are previously added in the state of solid solution during the manufacture of a nickel hydroxide, and the resulting nickel hydroxide is used as an active material for a positive electrode.
According to this method, deformation or expansion of the nickel electrode during the charge and discharge of the battery can be restrained. On the other hand, however, the discharge capacity of the battery which incorporates the nickel electrode as its positive electrode therein is reduced due to the effect of the added metal ions.
Thus, according to the method described above, the loadings of the metal ions should be limited in order to increase the discharge capacity. In this case, however, the deformation or expansion of the positive electrode cannot be restrained fully effectively. If the metal ion loadings are increased to restrain the deformation or expansion of the positive electrode, on the other hand, then reduction of the discharge capacity of the battery will result.